Buck regulators are a type of DC/DC controller that are used to convert a DC voltage to another lower DC voltage. The buck regulator chip is connected to an external LC circuit that includes an inductor and an output capacitor. An external voltage divider produces a feedback voltage. Typically, Pulse Width Modulation (PWM) signal(s) are created by the buck regulator to control the switching of an input voltage into the LC circuit. The PWM signal(s) may be composed of, or used to create, control signals for high-side and low-side switches for power and ground into the LC circuit to produce the DC output voltage. The larger the duty cycle of the high side PWM signal, the greater the DC output voltage.
The feedback voltage is compared to a constant reference voltage to adjust the PWM signal(s). If the feedback voltage, and thus the output voltage, is too low or high, the duty cycle of the high side PWM signal is increased or decreased.
Technology development for ceramic capacitors has made them the capacitor of choice for output capacitors in DC/DC converters. Energy density and cost reductions have made them practical and cost effective for output voltages of 5V and below. However, ceramic capacitors have a very low equivalent series resistance (ESR) and result in unstable operation with standard Constant On-Time (COT) control schemes.
Most COT DC/DC controllers rely on the ESR of the output capacitor to stabilize the buck system. The ESR of the output capacitor has dramatic phase correction effect and is capable of compensating for the phase lag of the buck regulator's LC circuit. When COT DC/DC controllers are used with low ESR ceramic capacitors, they have required either an additional resistor added in series with the output capacitor, a Resistor-Capacitor (RC) circuit feeding from the switch node to the feedback voltage, or a resistor in series with the inductor together with a RC circuit filter to stabilize the system. These extra components increase the cost, component count, and reduce efficiency. Further, when an additional resistor in series with the output capacitor is used (the simplest and cheapest method), the output voltage ripple increases and the transient response is degraded.